Labor Day this year brings little cheer to those its supposedly honors. The unions are on the defensive in their dealings with management.

They are losing political clout in the nation as a whole. Moreover, matters are likely to get worse, for if part of the trouble lies in labor's old and outgoing leadership, the main onus falls on the new incoming leadership of business.

Labor's plight begins where it hurts most-in the big, blue-collar unions that are the mainstay of the whole union movement. Not only are they declining as part of a work force that is steadily tilting toward the series and including more and more women; they are even losing ground within their shrinking turf.

The building trades-which, by virtue of their work on construction projects, used to be labor's foot in all doors-have recently been dealt a very harsh blow. The big construction companies have gone, as the saying goes, double breasted," While the companies maintain front firms that hire union workers, they have also developed subsidiaries that do the jobs with non-union labor. The upshot has been a drastic falling-off in the dominant role once played by organized labor in all construction. According to one calculation, 90 percent of the construction work done in the country was done by unionized workers only a couple of years ago. Now the total is under half.

The decline of unions in construction makes it that much easier for new plants to avoid hiring unionized labor once they get into production. The leading example is the largest industrial company in the country, General Motors.

After losing the great battle to the unions in the Depression years, GM came to assume that dealing with the United Auto Workers was a routine a part of making cars as putting in carburetors. In the last national contract negotiations, GM's top management signed a letter saying that it would do nothing to oppose unionization of new plants.

But the younger manages running the new plants, many of them in the Sun Belt states, have different ideas. They tend to recruit workers after the building trades, with their nucleus of unionized workers, have left the scene. They use questionnaires devised by professional psychologists to determine in advance which job applicants are most prone to joining unions. So by no mere accident 9 of the 10 plants opened by GM in the South since 1973 are non-union.

Labor has tried to shore up its declining bargaining position by political action. The common-situs-picketing bill was an effort to strenghten the hand of the building trades. But it was vetoed by President Ford and failed to pass the Congress last year.

The labor law reform bill was an effort to eliminate various abuses practised by some companies to avoid unionzation. It failed to win enough votes in the Senate to close off a filibuster.

On a host of other public issues, moreover, labor has been fighting losing battles. It has failed to make out of Humphrey-Hawkins a strong full-employment bill. Its support for national health insurance has proved of no avail in forcing early action. In the battle against inflation, the chief target of the past two administrations has been the unions-most recently the postal workers.

Much of the blame for labor's plight is heaped on the head of George Menny. the 84-year-old head of the AFL-CIO. It is ture that Meany, a plumber himself, has been slowto shift labor's emphasis from industry to the services; his bulldog countenance, gruff manner and Bronx accent accord ill with the modern image of the trim, well-spoken executive. The men around him have lagged in the development of newer lobbying techniques-especially the blitz of constituent letter and phone calls that did in common-situs picketing.

But whatever Meany's faults, the lion's share of the blame for putting labor up against the wall lies eslewhere. The new breed of managers, who knew not the Depression nor the war, are now making hay at labor's expense. They are cutting corners on collective bargaining and using the public dislike of taxes and government to kill humane causes dear to the unions. It is the whom we have to thank, if, as seems likely, labor now exchanges, a plave at the center of national consensus for an adversary role.